Splicing an optical fiber having twin cores

ABSTRACT

In joining an optical fiber containing twin cores to a corresponding optical fiber of standard type having a single core and having the same outer diameter, so that a core in the twin-core fiber is aligned with the core in the standard fiber, a conventional automatic splicing machine for optical fibers is used. The twin-core fiber is placed with its cores for instance located in a vertical plane. Then the fibers are spliced in a conventional manner with an alignment of the outer surfaces of the fiber ends and during the splicing procedure or from the finished splice the lateral offset of for instance the upper core of the twin-core fiber and the single core is determined. This can be carried out by capturing an image of the fiber splice in a heated state, when the splice is performed by fusion-welding. The determined offset value then gives the offset, which is to exist between the outer surfaces of the fiber ends for providing the correct alignment of the fiber cores. Hence, the splice region of the fibers is then removed and the fibers are spliced again, but this time with a suitable, calculated offset of the outer surfaces of the fibers.

The present invention relates to splicing a fiber having twin cores anda standard type fiber having a single core.

BACKGROUND

Standard type optical fibers comprise a cladding having an essentiallycircular cylindrical outer cladding surface and a thin fiber core, whichis located rather centrally inside the cladding and in the ideal case islocated along the longitudinal axis of the outer cylindrical surface andthus has the same longitudinal axis as this.

Optical fibers having twin cores, where each core has a shape similar tothe core of standard fibers, and the two cores are located essentiallysymmetrically along a diameter plane of the circular cylindricalcladding, constitute a material used in the research and utilization ofmany linear and non-linear phenomena, which are based on interaction ofthe evanescent fields of the fundamental propagational modes of thecores. Such phenomena are used in beam splitters, fiber sensors andnon-linear switches.

A major drawback associated with the use of such fibers is however thedifficulty both in the excitation and the detection of the signals inthe two cores due to their small size and due to the fact that they arelocated comparatively close to each other. A typical core radius in afiber having twin cores is approximately 3-4 μm and a typical distancebetween the two cores is of the magnitude of order of a few radii of thecore. It is impossible to obtain a butt joint between an optical fiberhaving a single core of standard type and a fiber having twin cores andbetween two fibers having twin cores by means of the conventionalsplicing methods.

A method, which has been used to overcome this problem, is to use largeand powerful optical elements and lenses to focus the incoming light tothe cores. Such methods, however, suffer from high losses (7-8 dB) atthe introduction of the light, which together with the inconvenience ofusing larger optical components, for instance due to theirunsatisfactory stability during practical use, make them inconvenientfor practical use.

In Swedish patent SE-C2 500 874, "Alignment of and splicing opticalPM-fibers", which is incorporated by reference herein, it is disclosedhow an optical PM fiber can be given a predetermined angular alignmentalong its longitudinal axis and how this alignment can be used toprovide good joints between two optical PM fibers. In the determinationthe fiber is illuminated with light and the lens effect is thenobserved, i.e. the light intensity is determined for light passingthrough the fiber. A light intensity curve perpendicular to the axis ofthe fiber has then generally a maximum, which corresponds to the core orto the central region of the optical fiber. Outside this maximum thereis a region having a lower light intensity, but where the lightintensity still can be fairly constant, on said line. Regions outsidethe outer surface of the fiber obtain a light intensity approximatelycorresponding to the light intensity without any fiber at all placed inthe light beam path. The lens effect is constituted by the contrastbetween the central region having a high light intensity and the regionwhich is located closest thereto. For an alignment the fiber is rotated,so that the lens effect either becomes maximal or minimal.

In the published International patent application WO-A1 95/14945,"Determination of angular offset between optical fibers having optical,axial asymmetry and alignment and splicing of such fibers", which isalso incorporated by reference herein, image analysis is used having arefined evaluation procedure of light intensity curves capturedperpendicularly to the longitudinal directions of the fibers in order tofor instance splice fibers having twin cores to each other.

SUMMARY

It is an object of the invention to provide a method and a device tojoin in a simple manner an optical twin-core fiber to a standard opticalfiber with an alignment of one core with the core in the standard fiberusing a minimum of extra devices to be applied in available automaticfiber splicing machines.

This object is obtained with the invention, the scope andcharacteristics of which appear from the appended claims.

In joining an optical fiber 1', see the schematic sectional view of FIG.12, containing twin cores 3' to a corresponding optical fiber 1 ofstandard type having a single core 3, a core in the twin-core fiber isto be aligned with the core in the conventional fiber, so that light inthis core in the twin-core fiber can also propagate in the standardfiber or vice versa. The offsets Δ₁ and Δ₂ between the outer sides ofthe fibers in the plane, that extends through the two cores 3' in thetwin-core fiber, is in this position given by

    Δ.sub.1 =(d-(D.sub.2 -D.sub.1))/2 and Δ.sub.2 =(d+(D.sub.1 -D.sub.2))/2

where D₁ is the outer diameter of the standard fiber, D₂ is the outerdiameter of the twin-core fiber and d is the distance between the coresin the twin-core fiber. If the fibers are of the same basic type, forinstance intended for single-mode-propagation of light of some suitablewave length, they can be assumed to have approximately the same outerdiameter, i.e. D₁ =D₂ and Δ₁ =Δ₂ =d/2, i.e. for a good coupling of lightin a core in one fiber to a core in the other fiber, the offset in saidplane shall be half the distance between the twin fiber cores. This isof course only exactly valid for the ideal case with well circularcylindrical surfaces, symmetrically located twin cores and a centrallylocated single core.

The distance d between the cores of the twin-core fiber can bedetermined by making a symmetrical test splice between the fibers and bydetermining the lateral offset between a core in the twin-core fiber andsingle core in this splice. Therefore the twin-core fiber must bepositioned in such an angular position along its axis, that the planethrough the cores in this position obtains a predetermined position.Therefore, capturing of intensity curves according to, for instance theabove cited International patent application WO-A1 95/14945 is used, anda determination of the height of the central peak thereof. This heightas a function of the rotational angle of the fiber has an extreme value,when the twin-core fiber is located having said plane perpendicular orparallel to a given viewing angle, for instance such as in FIG. 12 withthe plane perpendicular to the observation direction. Then the standardfiber is spliced to the twin-core fiber in this position, so that thefiber ends, as seen in this observation direction, are located oppositeto each other, having their axes aligned and having their outer sidesparallel in the conventional way, what is obtained by only observing theouter sides or outer edges of the fibers as viewed in this observationdirection, and then these sides are symmetrically placed in relation toeach other, see FIG. 13, so that the offsets (Δ₁, Δ₂) between the outersides of the fibers are equally large and have the same sign. Details ofthis placement procedure can for instance be taken from the Swedishpatent application SE-A 9100978-7, "Splicing optical fibers", filed Apr.3, 1991, which is incorporated by reference herein.

Practically, a conventional, automatic splicing machine for opticalfibers can be used. The twin-core fiber can then be positioned havingits cores for instance located in a vertical plane. During the splicingprocess or from the finished splice the lateral offset of for instancethe upper core in the twin-core fiber and the single core is determined.This can be carried out by capturing an image of the fiber splice in aheated state, a warm-fiber image, when the splice is made by means offusion-welding, as is described in the Swedish patent SE-C 469 200 citedhereinafter.

Thereupon, the splice region, where the fibers are joined to each other,is removed and the fiber ends are spliced to each other again but thistime with a suitable, calculated offset of the outer surfaces or sidesof the fibers. This offset can then somewhat exceed the earlierdetermined offset or the offset value, which is calculated according tothe above formulas, when the fibers have different diameters, where theexceeding amount is chosen with regard to the lateral movement of thefiber ends in a final welding of the fiber ends to each other. Thiscalculation and welding to each other can then be performed as isdisclosed in the Swedish patent application SE-A 9400781-2, citedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention, which is given for the purpose ofillustration only but is not intended to limit the invention, will nowbe described with reference to the accompanying drawings, in which

FIG. 1a is schematic view an optical fiber of standard type illuminatedby a light source and

FIG. 1b is a curve illustrating the light intensity when light haspassed through the fiber,

FIGS. 2a and 2b are a view and a curve similar to those of FIGS. 1a and1b for an optical fiber having twin cores for a first orientation of thetwo cores in relation to the direction of the incident light rays,

FIGS. 3a and 3b are a view and a curve similar to those of FIGS. 2a and2b for a second orientation of the two cores in relation to thedirection of the incident light rays,

FIG. 4 is a screen image, in which a relative central light intensity isgiven as a function of the rotation angle for a fiber having a twin coreand for a fiber having a single core,

FIG. 5 is a schematic picture illustrating the light beam paths and theelectrodes in a device for splicing optical fibers,

FIG. 6 is a schematic view, partly illustrated as a block diagram, of adevice for splicing optical fibers,

FIG. 7 is a flow chart of different operative steps performed in weldinga fiber having a single core to a twin-core fiber,

FIG. 8 is a screen image captured before the welding step fordetermination of a desired offset,

FIG. 9 is a screen image captured during the welding step fordetermination of a desired offset,

FIG. 10 is a screen image captured during the final welding of fiberends to each other,

FIG. 11 is a screen image captured after final welding to each other,

FIGS. 12 and 13 are schematic sectional views of fiber ends located nextto each other.

DETAILED DESCRIPTION

In FIGS. 1a, 2a and 3a the light beam path is schematically shown when aparallel light beam (coming from above as seen in the figures) passesthrough an optical fiber 1, 1' of standard type having a core 3, and ofa type having two cores 3' respectively, where the latter type isrotated to two different orientations around its longitudinal axis 7.The optical fibers 1, 1' have a cladding or cladding 5 having anessentially circular cylindrical outer surface, which encloses the cores3, 3'. The single core 3 of a standard fiber 1 is located approximatelysymmetrically inside the cladding 5, i.e. approximately concentricallythereto and consequently in such a manner that the longitudinal axes 7of the core 3 and the cladding approximately coincide. The cores 3' ofthe twin-core fiber 1' can normally be assumed to have approximately thesame diameter as the core 3 of the standard fibers and they are locatedat places in relation to the longitudinal axis 7 of the fiber 3', whichare located more or less exactly diametrically opposite to each other asseen in the cross section of the fiber, i.e. so that they are locatedapproximately in a plane 8 passing through the longitudinal axis 7 ofthe fiber 1' and symmetrically in relation to the longitudinal axis 7 ofthe fiber.

When joining a fiber of standard type having a single core to one of thecores of an optical fiber having twin cores, the end surfaces of thecores of the two fiber ends, which are to be spliced, are to be placedopposite to each other, in order to obtain a maximal transmission oflight from one of the fiber cores in one end of the twin-core fiber tothe fiber core in the end of the standard fiber.

In FIGS. 1b, 2b and 3b intensity curves of light, which has passedthrough the fibers 1, 1', are shown, the intensity curves being takenalong a direction perpendicular to the incident parallel light beam, andperpendicular to the longitudinal axis 7 of the optical fiber. Further,the curve is captured along a line, which extends approximately throughthe focal line for the cylindrical lens, which is formed by the cladding5 of the fiber. For a standard fiber 1 having a single core 3, forinstance of single-mode type, a rather high central peak 9,corresponding to the longitudinal axis 7 of the fiber, and next to thispeak regions 10 having a relatively constant, low light intensity, areobtained as shown in FIG. 1b. Outside these regions 10, having a lowlight intensity, there are regions having a constant higher intensity,which is the kind of light, which has travelled past the fiber and isbasically unaffected thereof. The transition to these regions thencorresponds to the outer sides or surfaces of the fiber, as seen in theincident direction of the light, and the very border or transition tothese outer regions can be used for positioning the fiber in atransverse or lateral direction, see below.

In FIG. 2a an orientation of an optical twin-core fiber 1' is shown, inwhich the fiber cores 3' are located, so that they are both locatedaligned with and symmetrically in relation to the direction of incidentbeam. Light rays, which are incident to the cores 3' do not materiallycontribute to the light intensity, which can be observed on the otherside of the fiber, i.e. after the light rays passing through the fiber1'. Deflections of the light rays occur namely during their travel intoand out of these regions and at reflections to the surface of the fibercores. Since a cylindrical body, as mentioned above, has a focusingeffect on incident parallel light rays, what is moreover termed the lenseffect of the optical fiber, an increased light intensity at thecorresponding focus is obtained, which in the light intensity curve isshown as a central, not very high peak 9, as illustrated in FIG. 2b.

In FIG. 3a the orientation of the optical fiber 1' is instead such, thatthe two cores 3' are essentially located along a diameter 8 of theoptical fiber 1' being perpendicular to the direction of the incidentparallel beam. As appears from the figure, in this case some of theouter incident light rays are indeed prevented from passing through theoptical fiber due to the interfering effect of the cores 3', but centrallight rays coming from directions, which are located next to and closeto the fiber axis 7, pass therethrough. A corresponding light intensitycurve, which to a considerable amount is formed by the central rays andwhich is shown in FIG. 3b, has a central peak 9, which has aconsiderably larger height in this case compared to the curve of FIG.2b.

In a continuous rotation of the optical fiber 1, 1' around itslongitudinal axis, for each angular position, a curve of the type, whichis shown in FIGS. 1b, 2b and 3b is obtained, and in which the centralintensity peak 9 has a varying height h above the adjacent parts of thecurve. For the twin-core fiber 1' FIGS. 2b and 3b show two extreme casesof the curve, so that at other rotational positions of the twin-corefiber height values h are obtained, which are in the range between theheight values h for the curves illustrated in these figures. In all ofthese curves this value h, which thus constitutes the difference betweenthe height of the central peak 9 and the directly surrounding portions10 of the light intensity curve, determined as has been described above,is determined for rotations of the fiber 1, 1' at different angularpositions.

This value h is determined for different angular positions of theoptical fiber 1, 1', for instance for each tenth degree. Curves of suchdetermined height values h appear from the photograph of FIG. 4 of ascreen image, which has been calculated by an automatic splicing device,which will be described hereinafter, and which is shown by the monitorthereof. In the top portion of FIG. 4, the values h for a twin-corefiber at different rotational positions are hence shown by the brightpoints and in the bottom portion of FIG. 4 the corresponding heightvalues for a fiber having a single core are seen. The values in thebottom portion are rather constant for different rotations, since thefiber in this case is essentially rotationally symmetric, of coursehaving some small deviations from a circular shape and concentricpositions, which give the small variations in the values for differentrotation angles. The values for a twin-core fiber, however, have largevariations for different angular positions, having extreme valuescorresponding to fiber positions according to FIGS. 2a and 3b. Inparticular, the values h for the twin-core fiber present also largedifferences for fibers of different manufacture and design, for instancedepending on the distance between the two cores 3', the diametersthereof, refractive index gradients, etc.

Essential optical components in a commercially available splicing devicefor optical fibers of standard type are shown in FIG. 5 with a standardfiber 1 having a single core and a twin-core fiber 1' mounted in thedevice. Two light sources 11 are arranged in such a way that they emitlight beams, which reach the end regions of the optical fibers 1, 1'under mutually perpendicular directions, the x- and y-direction,respectively. The light beams passing through the fibers are deflectedby means of mirrors 13 and are focused to one single parallel beam bymeans of a beam splitter 15. The single parallel beam obtained in thismanner hits the light sensitive elements of a video camera 17 ofCCD-type, which is connected to a monitor 19. Lens systems, not shown,can be arranged at different locations of the light beam path in orderto obtain sharp reproductions.

The mechanical and electronic components in such a device for splicingtwo optical fibers are schematically shown in FIG. 6. This device isfundamentally a conventional automatic splicing device for opticalfibers supplemented with devices for orientating the fibers angularlyand having special routines for analysing the determined light intensitycurves.

The two optical fibers 1 and 1', which are to be spliced, are placedwith their ends in special retainers 27, by means of which the fiberscan be rotated about their longitudinal axes. These retainers 27 arefurther arranged on conventional alignment supports 29 for the fiberends of the splicing machine. Further, the fiber supports 29 can beoperated in relation to each other, in the same perpendicular directionsx and y, which are indicated by the two light beam directions from thelamps 11 in FIG. 4, and also in the longitudinal direction of the fibersby means of drive motors 31, which are controlled by logical circuitsand software in a processor logic module 33. The lamps 11 are activatedby their own driver circuits 35 of the processor logic 33. Weldingelectrodes 25, at the points of which the fiber ends are to be placed,are energized by their driver circuits 37 controlled by the processorlogic 33. The video camera 17 captures the image of the fiber ends andprovides the corresponding video signals through a video interface 41 toan image processing and image analysis module 43. The result of theimage processing and the image analysis in this module 43 is sent to theprocessor logic module 33 and the result can be shown on the monitor 19.Also the directly obtained image of the end region of the fiberscaptured by means of the video camera 39 can be shown on the monitor 19.

In measurements and a possible splicing of two optical fibers, they areplaced having their ends in the rotatable retainers 27, so that thefibers are aligned in parallel with and opposite to each other. By meansof the conventional control by means of the processor logic module 33,the two fibers are aligned with each other in the transverse directionin relation to the longitudinal axes of the fibers and their endsurfaces are also placed close to each other. An image of the end regionof the fibers can then be shown on the monitor 19 and by means of theimage processing and image analysis module 43 also curves correspondingto the curves of FIGS. 1b, 2b and 3b are determined for severaldifferent straight lines, which are equally spaced from each other andare perpendicular in relation to the longitudinal direction of thefibers on each side of the planned splice.

By operating knobs 47 of the rotatable retainers 27 the rotational angleof the fibers are varied from an initial position, so that curves arecaptured for equally spaced angular values over a full revolution, forinstance as suggested above for each tenth degree. The heights h oflight intensity profiles for a certain angular position are determinedby an automatic analysis of the curves, the height of their centralpeaks are determined and the average of several such heights is thencalculated. The corresponding numerical values for each fiber end canthen be continuously shown on the monitor 19. When the fibers are turnedor rotated by means of the operating knob 47 it might still be necessaryto adjust the position of the fibers in order for them to be observableand this is performed as before by the automatic alignment control inthe processor logic module 33 by energizing the control motors 31 forthe retainers 29.

In FIG. 7 a method is illustrated by means of a block diagram, forsplicing a twin-core fiber to an optical fiber of standard type having asingle core in which the splicing is performed with an alignment of acore in the twin-core fiber with the core in the standard type fiber, sothat the obtained splice in the normal case gets an acceptably lowattenuation. The procedure is started in a step 701 by inserting thefiber ends of the optical fibers 1 and 1' respectively into the fiberretainers 27 in the automatic splicing device as described above. Theapparatus is then started by operating some suitable operating key orsimilar device, not shown, so that the execution start of a programwhich is present in the processor logic part 33. Hereupon for instancefirst the two lights sources 11 are ignited for emitting light beams inthe x- and y-directions. In a step 705 a rough alignment in thetransverse directions in relation to the fibers is performed in theconventional manner, i.e. an alignment of the fibers in the x- andy-directions, and further the end surfaces of the fiber ends are broughtto a position close to each other, centrally between the points of theelectrodes 25. An image is captured in this step and is automaticallyanalyzed by the image processing and image analysis part 43 in order tomake certain that the fiber ends are well cleansed and that there are noremaining particles on the outer surfaces of the fiber ends. Ifparticles of dirt should be present, a suitable message can be shown onthe monitor and the procedure be stopped until the fiber ends have beencleansed and again been placed in the retainers.

Otherwise the image obtained in this position is analyzed in a step 707for making certain that the end surfaces are located in a right angle orat least in angles very close to a right angle in relation to thelongitudinal direction of the fiber ends. Here it is also checked thatthe longitudinal directions of the fiber ends, i.e. the outer surfacesof the fiber ends, as appearing on the monitor 19, are located wellhorizontally in the image, i.e. that they have the position appearing inparticular from the screen image on the monitor 19 of FIG. 5. If thesedifferent checks should give a negative result, some suitable message isshown on the monitor, whereupon an operator can take proper actions,such as releasing a fiber end and again perform a cutting of the end inorder to obtain a cut surface, which is located in a more perpendiculardirection.

After this curves of the type, which is shown in FIGS. 1b, 2b, 3b and inFIG. 4, are to be captured, and therefor an accurate transversealignment of the fiber ends with each other is performed in a step709--the earlier mentioned transitional locations in the captured imageare then used, which represent the positions of the outer surfaces ofthe fibers--and further the end surfaces of the fiber ends are placednext to each other in a position suitable for welding them to eachother, by the method that the processor logic provides suitable signalsto the driver circuits for the drive motors 31 during a simultaneousimage processing of image analysis of the image captured by the videocamera 17. As is conventional, this positioning is performed with afeedback control through a continuous image analysis and feedback of theresult of the image analysis to the processor logic 33, which controlsthe driver circuits 31 for obtaining the accurate alignment. The endsurfaces of the fiber ends then become placed exactly centrally betweenthe ends of the electrodes 25. In a step 711 then a refocusing isperformed, so that a maximal lens effect is obtained, i.e. as largelight intensity as possible in the central longitudinal region. It ismade by moving a main lens, not shown, belonging to the video camera 17.After this a step 713, in which at least the end of the twin-core fiberis turned in suitable, equally large steps, for instance of the size10°, around its longitudinal axes, is performed in order to capturecurves of the type illustrated in FIG. 4. Strictly, only curvescorresponding to the half of a full turn, i.e. 180°, are required, sincethe twin-core fiber can be assumed to have a two-fold axial symmetry.For obtaining a better measurement significance, however, preferablycurves for a full turn are obtained.

After this in a step 715 the fiber 1' having twin cores is rotated tothe position shown in FIGS. 2a and 5, in which the maximal value of theheight h is obtained in a light intensity curve as seen in thex-direction and which has been determined by an evaluation of thecaptured curves for different rotational angles. After this, analignment is to be performed only with regard to the outer surfaces ofthe fiber ends, i.e. the outer surfaces of the cladding 5, and thereforthe focusing of the obtained image on the video camera 17 is againadjusted in a step 717 by controlling the lens system for the videocamera, so that these outer surfaces are sharply reproduced on the lightsensitive surface of the video camera 17.

If it is now presumed that the fiber 1' having twin cores is of anunknown or new type, the distance between its two cores 3' must in someway be measured or determined. Thus, in this case there are no such datastored in the processor logic module 33 in the splicing machine. Thedecision that this case is at hand is made in a block 719, and hence ifno such data are stored for the fiber 1' having twin cores, a finealignment of the outer sides of the claddings of the fiber ends isperformed in a step 721, like in the block 709, and also an accuratealignment in the longitudinal direction, so that the end surfaces of thefiber ends are placed in the position for being welding to each other,centrally between the points of the electrodes 35. A screen imagecaptured of this step is shown in FIG. 8.

After this the fibers are welded to each other in a normal manner, as isindicated in a block 723, for instance as is described in the Swedishpatent SE-C 469 200, "Method and device for splicing optical fibers anddetermining the loss/attenuation in a splice of two optical fibers",which is incorporated by reference herein. As also appears from thispatent an image can be captured of the joint between the two fibersduring the welding process and from this image the lateral offset of thefiber cores in relation to each other in a considered direction appears.This is performed by the image analysis module 43 in a step 725. In ablock 727 the value of the lateral offset determined in that way betweenin this case the upper core in the twin-core fiber 1' and the singlecore in the standard fiber 1 is stored in a memory in the processorlogic part 33 to be used, when a correct splice is to be performedbetween the same fibers or fiber types. An image captured of the fiberends in a heated state during the welding process is also stored in amemory and this image together with the calculated numerical value ofthe offset between the fiber cores are shown on the monitor 19. Afterthis the procedure is terminated in a block 729 and the device, togetherwith lamps, driver circuits, etc. is turned off.

Thus, a standard value of the lateral offset has now been determinedbetween a core in the twin-core fiber 1' and the core in the standardfiber 1, when the claddings are aligned with each other, which value isessentially equivalent to half of the distance between the two cores inthe twin-core fiber, since it can be assumed that at the set rotationalangle of the twin-core fiber, this fiber is located always in the sameposition having one core at the top and the other core locatedvertically below. If the same fiber is now to be spliced in a correctmanner with an alignment between the upper fiber core of the twin-corefiber and the core in the standard fiber, the earlier performed spliceregion and the region of the fiber ends adjacent thereto are to beremoved, by cutting the fibers at suitable locations and preparing theirends in the usual manner for splicing.

Then the procedure starts again according to the flow chart in FIG. 7and the steps 701 to 719 are executed in the same manner as above. Inthe step 719 one will now instead find that data values are stored forthe twin-core fiber and therefore, a block 731 is performed after theblock 719 in this case. In this block a transverse fine alignment, inthe x- and y-directions is performed, compare the block 721, so that thefiber ends are here placed with a predetermined calculated, suitableoffset in relation to each other. This offset then somewhat exceeds thestored offset value. Further, a positioning of the fiber ends in thelongitudinal direction is performed, so that their end surfaces areplaced close to each other centrally between the points of theelectrodes 25. Thereupon in a block 733 welding of the fiber ends toeach other is performed with an indirect check that the previouslydetermined offset between the fiber cores, as stored in the block 727,is obtained through a continuous observation of the outer side of thefibers, so that the value of the offset of the outer sides will agreewith the stored offset value. The calculation of a suitable initialvalue for the alignment, the continuous alignment itself, during theheating and welding to each other in the blocks 731 and 733 areadvantageously performed in a manner, described in the Swedish patentapplication SE-A 9400781-2, "Controlled splicing of optical fibers",filed Mar. 8, 1994, which is incorporated by reference herein.

Further, like in the block 727, an image is captured of the joint duringthe very welding process and the image is automatically analyzed by theimage processing and image analysis part 43 in order to evaluate theloss in the obtained joint through an evaluation of a possible offset inthe image between the upper fiber core and in the core of the standardfiber, as is described in the Swedish patent SE-C 469 200, cited above.An image captured of the fiber ends in a heated state is also stored andis shown on the monitor 19, see the picture of FIG. 10, from which thealignment of the fiber cores appears. In a block 737 the evaluatednumerical value of the loss in the fiber splice is then shown on themonitor 19 and also an image of the fiber splice in a cooled state, asis seen in FIG. 11, where the offset of the outer sides of the fibercladdings clearly appears. The bright line in the middle of the fiberends does not show the fiber cores but corresponds to the central peak 9of the lens effect, see FIGS. 1b, 2b and 3b. Then the procedure ends andthe block 729 is performed as above.

What is claimed:
 1. A method of determining the distance betweensymmetrically placed cores in an optical fiber having twin cores, themethod comprising the steps of:positioning an end of the optical fiberhaving twin cores to allow images to be captured of the end in adirection substantially perpendicular to a longitudinal axis of the endand substantially perpendicular to a plane through longitudinal axes ofthe twin cores within the end, providing an optical fiber having asingle, centrally located core, splicing the end of the optical fiberhaving twin cores to an end of the optical fiber having a single corewith a symmetrical or concentric positioning of outer sides or surfacesof the ends in relation to each other and with the outer sides orsurfaces of the ends parallel to each other, the splicing beingperformed by heating and welding the ends to each other, capturing inthe direction an image of the ends when being heated, the cores of thefibers being visible in the image, determining from the image a value ofthe offset of one of the twin cores in the fiber having twin cores andthe core in the fiber having a single core, and calculating a valueequal to twice the determined value of the offset and taking thecalculated value as the distance between the cores in the optical fiberhaving twin cores.
 2. The method of claim 1, wherein in the step ofpositioning the optical fiber having twin cores, further comprising thesteps of:rotating the optical fiber having twin cores about itslongitudinal axis, capturing images during the rotating in a directionsubstantially perpendicular to the longitudinal axis of the end of theoptical fiber having twin cores and for a multitude of rotation angles,evaluating the images to find a rotation angle for which a maximumexists in the image captured for this rotation angle of the differenceof the light intensity in portions of the image corresponding to thelongitudinal axis of the end and portions corresponding to regions inthe image located at both sides of the longitudinal axis, and rotatingthe optical fiber having twin cores to the found rotation angle.
 3. Amethod of joining an optical fiber having twin cores locatedsubstantially symmetrically to an optical fiber having a single,substantially symmetrically located core, so that an alignment of one ofthe twin cores in the optical fiber having twin cores is obtained withthe core in the optical fiber having a single core, the methodcomprising the steps of:determining the distance between the twin coresof the optical fiber having twin cores, positioning an end of theoptical fiber having twin cores with an end surface next to an endsurface of an end of the optical fiber having a single core, so that theouter sides or surfaces of the ends are located in parallel, the offsetbetween the outer sides of the fiber ends, as taken in a directionperpendicular to longitudinal directions of the ends and in a planethrough the longitudinal axes of the twin cores of the end of theoptical fiber having twin cores, has a value, which is determined fromthe distance between the twin cores in the optical fiber having twincores and from a difference between the outer diameters of the opticalfibers, and joining, after positioning, the ends to each other, whereinthe step of determining the distance further comprises the stepsof:positioning an end of the optical fiber having twin cores to allowimages to be captured of the end in a direction substantiallyperpendicular to the longitudinal axis of the end and substantiallyperpendicular to the plane through the longitudinal axes of the twincores within the end, providing a test optical fiber having a single,centrally located core, splicing the end of the optical fiber havingtwin cores to an end of the test optical fiber having a single core witha symmetrical or concentric positioning of outer sides or surfaces ofthe ends in relation to each other and with the outer sides or surfacesof the ends parallel to each other, the splicing being performed byheating and welding the ends to each other, capturing in the directionan image of the ends when being heated, the cores of the fibers beingvisible in the image, determining from the image a value of the offsetof one of the twin cores in the fiber having twin cores and the core inthe fiber having a single core, and calculating a value equal to twicethe determined value of the offset and taking the calculated value to bethe distance between the twin cores in the optical fiber having twincores.
 4. The method of claim 3, wherein in the step of positioning theoptical fiber having twin cores, further comprises the step of:rotatingthe optical fiber having twin cores about its longitudinal axis,capturing images during the rotating in a direction substantiallyperpendicular to the longitudinal axis of the end of the optical fiberhaving twin cores and for a multitude of rotation angles, evaluating theimages to find a rotation angle for which a maximum exists in the imagecaptured for this rotation angle of the difference of the lightintensity in portions of the image corresponding to the longitudinalaxis of the end and portions corresponding to regions in the imagelocated at both sides of the longitudinal axis, and rotating the opticalfiber having twin cores to the found rotation angle.